1. Technical Field
The process of the present invention is directed to a process for electroplating tin on metal substrates.
2. Art Background
Tin is known as a metal that has excellent corrosion resistance. Good soldered connections are formed on tin surfaces because an excellent bond forms between the tin and the solder. Furthermore, when tin is plated on metals such as steel, copper, aluminum, nickel, and alloys thereof, the tin plate provides corrosion resistance and solderability to these metal substrates. Tin plate coatings are typically soft and ductile.
The electrolytic tinning of steel strip to produce tin plate is known. Tin is plated on steel using high speed electroplating processes. In high-speed processes, the substrate passes through the electroplating bath quickly and the bath itself is subjected to vigorous agitation and solution circulation. High speed process for electroplating tin onto substrates is described in U.S. Pat. Nos. 4,994,155 and 4,880,507 to Toben et al. A high speed process for electroplating tin onto steel strips is described in U.S. Pat. No. 5,174,887 to Federman et al. In these processes, electroplating occurs in a bath that includes a basis solution of either an alkyl sulfonic acid or an alkoyl sulfonic acid. The bath also includes a surfactant that does not foam during electroplating. Low foaming surfactants are used to prevent overflow from the reservoir tank. Such overflow is detrimental because it wastes solution. Foam also adversely affects the pumps used to agitate and circulate the solution.
Electroplating bath compositions that are used in high speed electroplating processes must contain chemical constituents that are stable when subjected to the temperature and degree of agitation required for high speed electroplating. Furthermore, the chemical constituents of such baths must be stable when exposed to air, since the bath is open to ambient air during the process. The bath must be clear and free from turbidity over a wide range of temperatures and should therefore have a cloud point (i.e. the temperature below which the chemistries remain clear) above 54xc2x0 C. (130xc2x0 F). Baths should also operate at current densities at or above 500 ASF. Finally the electrodeposits provided by such baths should be highly reflective and be relatively defect free and remain defect free after being subjected to reflow conditions. Electrolytic bath chemistries that satisfy these conditions are sought.
The invention is an electrolytic bath, which is useful for electroplating tin onto steel under high speed electroplating conditions. High-speed electroplating conditions are described in U.S. Pat. No. 5,174,887 to Federman et al., which is hereby incorporated by reference. High-speed conditions for electroplating tin onto steel, and the apparatus used therefor, are well known to one skilled in the art and are not described in detail herein.
The electrolytic plating bath of the present invention is an aqueous solution of an alkyl sulfonic acid and a solution soluble tin compound. An example of a suitable alkyl sulfonic acid is methanesulfonic acid. An example of a solution soluble tin compound is tin methanesulfonate. The concentration of the alkylsulfonic acid in the bath is selected to provide a bath pH of about zero to about three. Concentrations of alkylsulfonic acid in the range of about 10 g/l to about 30 g/l are contemplated as suitable. Suitable concentrations of tin (as metal) are about 10 g/l to about 100 g/l. It is advantageous if the tin concentration is about 10 g/l to about 30 g/l. Although higher concentrations (i.e. concentrations as high as 100 g/l) of tin are technically feasible, these higher concentrations are not viewed as advantageous economically.
The solution also contains at least one organic additive that is the reaction product of a polyalkylene glycol and phenolphthalein and derivatives of phenolphthalein. In the reaction product, the number of alkylene oxide moieties (e.g. xe2x80x94CxHYOxe2x80x94 wherein y equals 2x) is about 10 to about 62. The alkylene oxide moieties can be substituted or unsubstituted. In the embodiment of the present invention wherein the polyalkylene glycol is ethylene glycol, the alkylene oxide moieties are ethylene oxide moieties.
The condensation product is formed by reacting either a polyalkylene glycol with the phenolphthalein or phenolphthalein derivative. In one embodiment, the condensation product is formed before it is added to the electrolytic plating bath. In a second embodiment, the condensation product is formed in the electrolytic plating bath by separately adding to the bath: 1.) the alkylene glycol or polyalkylene glycol; and 2.) the phenolphthalein or phenolphthalein derivative.
The alkylene oxide is either substituted or unsubstituted. If substituted, methyl and ethyl substituents are contemplated as suitable. It is advantageous if the concentration of the organic additive (as condensation product) in the solution is about 0.2 g/l to about 2 g/l.
The solution also contains an antioxidant. Examples of suitable antioxidants include a sodium salt of benzaldehyde sulfonic acid and derivatives thereof. The concentration of the antioxidant in solution is about 0.15 g/l to about 1.5 g/l.
In the process of the present invention, the previously described plating solution is prepared by adding either stannous alkyl sulfonate or stannous alkoyl sulfonate to an aqueous solution of alkyl sulfonic acid or alkoyl sulfonic acid. Assuming 1 liter of the solution is being prepared, about 10 g to about 30 g tin (added as the stannous sulfonate) is added to one liter of an aqueous solution that contains about 10 g/l to about 30 g/l of alkyl or alkoyl sulfonic acid. To this solution is added at least about 0.2 g/l of an organic additive that is the condensation product of an alkylene glycol or polyalkylene glycol and phenolphthalein or phenolphthalein derivative. Phoenolphthalein has the following structure: 